Exposure itself is a pretty simple idea – your camera (assuming it is digital) has a sensor buried inside that is light-sensitive. When you take a picture, you expose the sensor to light for a given period of time. However much light gets in determines the brightness (or darkness) of the resulting image.
Of course, in real life, simple things aren’t always made simple. In general, there are three things that control exposure – and these things make up something called the “Exposure Triangle”:
- Shutter Speed
- Aperture (f/stop)
- ISO (sensitivity)
Each one of these items together determines your exposure. If you change any one without changing the others, you will see radically different exposures because all three work together. Each of these items also has an impact on other factors in the image (such as noise, depth-of-field, and stopping/blurring the action), which we’ll cover in a post on each (Each one could fill a book!)
Quickly, though, each of the items above help determine the exposure in different ways. “Shutter Speed” controls how long the shutter is open – thus controlling how long the sensor in your camera is exposed to light. If the shutter speed is short, then only a little light can fall on the sensor. Conversely, if the shutter speed is long, more light can fall on the sensor. However long your shutter needs to stay open depends upon a few factors, namely:
- The amount of light available (Is it day? Is it night?)
- How still the camera is (are you hand-holding? Is the camera mounted on a tripod?)
- Your Aperture and ISO settings
What matters most is that for the time the shutter is open and exposing the sensor, the camera itself should not move (unless you are after some artistic effects). This also applies to the scene the camera sees – if anything moves while the shutter is open, it turns into a blur. (And sometimes this is what you want. But not usually.) The biggest cause of blur is camera shake – the fact that we humans aren’t very stable and can’t hold anything still for a long period of time – thus, if we are handholding, a fast (short) shutter speed is a good idea.
A good rule of thumb: the shutter speed shouldn’t exceed the inverse of your focal length. For example – if my lens is at 50mm, then my shutter speed shouldn’t be longer than 1/50s. If I’m zoomed in at 200mm, then I can’t go slower than 1/200s. But if I’m zoomed out at 18mm, I can get by with 1/18s. If you want to be safe, halve the amounts – at 18mm, don’t go longer than 1/30s, at 50mm, don’t go longer than 1/100s, and at 200m, don’t go longer than 1/400s.
Aperture (or f/stop)
The next factor is the "Aperture”, or “f/stop”. This controls the amount of light allowed to get through the lens – think of it like a pipe. A large pipe can let a lot of water through, but a small pipe can only let a little light through. The same holds true for the aperture. A large aperture lets a lot of light through, but a small one only lets a small amount through. We usually express the size of the aperture in terms of “f/stops”.
f/stops are really a ratio between the size of the aperture and your focal length. Because of this, things get a little wonky when talking about them – especially for beginners. This is because a small f/stop means a large aperture, and a large f/stop means a small aperture. The benefit of wrapping your head around this is that f/8 is the same as f/8 on any camera and lens– it lets relatively the same amount of light through to the sensor.
The scale of f/stops is usually listed as it appears below. The important thing to remember is that increasing by one f/stop is the same as halving the amount of light that can get through to the sensor. Conversely, decreasing by one f/stop is the same as doubling the amount of light that can get through to the sensor.
The F/stop Scale: f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, …
Most cameras let you get in-between the stops – usually by 1/3 or 1/2 stops. Repeating the scale above including all the 1/3 increments gives us this list:
f/1.0, f/1.1, f/1.2, f/1.4, f/1.6, f/1.8, f/2, f/2.2, f/2.5, f/4, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22, …
Just remember that the smaller the f/stop, the more light is entering the camera, and vice versa. Also remember that your lens may or may not be able to reach the lowest (or highest) f/stops – this is usually indicated in the manual for your lens (or camera).
There are creative reasons you might want to use a small f/stop, vs. using a large f/stop, but we’ll cover these in a later post.
The last item is “ISO” – how sensitive your sensor is to light. In the digital world, this is really just amplification – think of recording a soft-spoken individual, then amplifying the recording to make them louder. The problem with amplification is that you don’t only amplify the person, but you also amplify everything else, including background noise. What was once a nearly inaudible hiss may quickly become a mighty roar, quickly obscuring the words of the speaker.
The same applies to the ISO setting in your digital camera. The lower the ISO, the less noise. The higher the ISO, the worse the noise gets, to the point of obscuring detail in the image. Most cameras offer a range of ISO 100 – 1600. (Some go as low as ISO 50; several can go higher – like ISO 6400). The typical ISO range is expressed like this:
ISO: 50, 100, 200, 400, 800, 1600, 3200, 6400, …
While most cameras only let the user pick from one of these settings, when using “Auto ISO”, the camera is free to pick any ISO setting, including ones in-between. (Which explains why you might see a picture at ISO 320 or ISO 64 instead of ISO 400 or ISO 50.)
So – you might ask – if using the lowest ISO generates the best image quality, why would I ever use anything else? The answer is that if you are shooting in low light (at night, or even just indoors), a low ISO may make your shutter speed too long, and you may not be able to get a sharp picture – so if you are hand-holding, or taking pictures of a sports event and need to freeze the action, you need to use a fast shutter speed – which means increasing your ISO (assuming you’ve already set your aperture as wide as it will go). It quickly becomes a trade-off of image quality (noise) vs. getting the image at all. It also depends on your camera – a dSLR with a large sensor will generate less noise at high ISO settings than will a point-and-shoot with a small sensor with the same number of megapixels as the dSLR.
Marketing Alert: Big numbers sell – fact of life. Unfortunately, camera marketers have latched on to the “megapixel” as the “big number” – which means that most people think a 15-megapixel camera generates better images than a 9-megapixel camera. This is, unfortunately, not true, because it really boils down to the number of megapixels and the size of the sensor. If the sensor remains the same size, increasing the megapixels will result in a noisier image – not a better one. This is why older point-and-shoot cameras can often out-perform their younger siblings when in low-light situations – less megapixels, meaning less noise. It is also why professional photographers buy dSLR or even medium-format cameras – a bigger sensor means more room for those megapixels, and thus less noise. (Of course, newer cameras offer their advantages – better battery life, better screens, HD video, etc. Just don’t let the number of megapixels be your only reason for picking a camera.)
Above: Example of increasing ISO and the resulting increase in Noise and loss of Detail
Camera: Canon Powershot SX110IS, 9.0 Megapixels, 1/2.3” sensor. A dSLR would
generate significantly better results at high ISO.
There is one other item that I didn’t include in the list above: “Exposure Compensation”, or “EV Comp”. The reason this exists is simply because the camera has to guess at the right combination of settings to use in order to get a “properly” exposed image (one where the subject is not too bright, but not too dark either). Sometimes the camera gets it wrong, and the photographer needs to compensate – and it is here they use “Exposure Compensation”.
Essentially this controls the shutter speed – so the photographer could just adjust the shutter speed for the same effect – but not all cameras make this easy, practical, or even possible. (Especially point-and-shoot cameras.) Instead they make it easier to use the Exposure Compensation button (usually a +/- button). Usually a camera will offer a range of -2 stops (four times darker) to +2 stops (four times brighter) with 1/3 stops in-between.
Above: Example of Exposure Compensation, from -2 to +2 EV.
Camera: Canon Powershot SX110IS, 9.0 Megapixels, 1/2.3” sensor.
One Final Word about Exposure
I lied. Ha!
If you’ve read this far without your brain exploding, congrats! And if you haven’t, then come back again in a few days and try to read a little further. It gets easier, I promise!
As far as exposure goes, we do have a few terms we like to pair with the word in the photography world. Understanding them will help you understand what everyone else is talking about:
- “Under”-exposed: The subject in the image is too dark. See the above image – “-2EV” is definitely under-exposed.
- “Over”-exposed: The subject in the image is too bright, and may have lost detail due to “highlight clipping”. See the above image – “+2EV” is definitely over-exposed, and the whites of the subject have lost significant detail, meaning that detail proably can’t be recovered - (hence the term, highlight clipping).
- “Correct” exposure: while this is in the eye of the beholder (if the image is what you wanted it to be, it’s correctly exposed), this would mean the subject isn’t too bright or too dark. In the above image, anything between “0EV” and “1EV” could be considered to be correctly exposed, depending upon what I, the photographer, wanted.
So there you have it – almost. There’s an important detail I’ve left out in this discussion, and that’s called “metering modes”. There are several different modes, and each controls how the camera sees a scene. Nearly every camera lets you change the mode you use, but suffice to say that there are generally three modes on most cameras (some have four or more):
- Evaluative, Matrix, Pattern
- Center-weighted Average
The Evaluative/Matrix/Pattern mode essentially looks at the entire scene, goes through a big database, and determines the best settings to use based upon what it sees. If the top of the image is bright, the bottom is dark, and there’s a spot in the middle that is in-between, chances are you’re photographing a person against bright sky and dark landscape – and the camera picks settings that will make the person not too dark and not too bright.
The Center-weighted Average mode takes the center of the image and weights it above everything else in the image. Here the camera assumes that the center of the image is what you are exposing for, and adjusts the settings appropriately to keep whatever is in the center not too bright, and not too dark.
The Spot mode takes a tiny portion of the center of the image and bases all the settings on that tiny little area. If the area is dark, but the rest of the image isn’t, you’ll end up with the dark area properly exposed, but the rest will be way too bright. (And vice versa, if the area is bright, but the rest isn’t, you’ll end up with the bright area properly exposed, but the rest will be dark.) Not too long ago, this was how all cameras that did automatic exposure worked – but now the Evaluative/Matrix/Pattern mode is generally the best choice. (Really – lots of research, effort, and time has gone into making the evaluative/matrix/pattern modes work well – definitely more time than you yourself have spent figuring all this out.) About the only time you’d use Spot now is if you were taking pictures of small bright lights (either candles, Christmas tree lights, etc.), and wanted everything but the lights to be ignored.
Ok – I’m done, now, I promise!
Well – until the next time! None of this is very hard to figure out, but it does take time and lots of practice. But that’s a benefit of this digital world – if you take a picture and don’t like it, it didn’t cost you anything but a few seconds to create. You can delete it, try another setting, and try again, all without wasting money on film, developing, and printing. And that’s the only real way to improve your photographs – take lots of photos, practice with your camera settings, and experiment.
And so, until next time – keep writing with light!